The present invention relates to both ceramic and metallic open-cell foams for a wide range of applications, and processes, in particular continuous processes, for their preparation.
Inorganic foams are known per se. They are prepared by a wide range of processes with so far insurmountable disadvantages and therefore costs which are so high that said processes are used only in special cases. By far the most widely claimed process comprises infiltrating open-cell polymer foams with a slip which contains inorganic particles. The infiltrated polymer foam, generally a polyurethane foam, is carefully dried, the organic components are removed by slow controlled heating and the negative consisting of inorganic powder is sintered. This itself is the reason for the complicated, expensive preparation. Both the drying of the pore structure filled with slip and the removal of the organic components by pyrolysis are very time-consuming. In addition, the material thicknesses are limited to a few centimeters owing to the slow drying and pyrolysis. The preparation of such foams is described, for example, in DE-A 39,34,496 or EP-A 157,974. EP-A 440,322 describes the complicated technology for the preparation of open-cell ceramic foams by means of an arrangement of rollers for infiltration and compression of the infiltrated polymer foams.
A wide range of applications have been described for inorganic foams, owing to their high-temperature stability and resistance to media. Thus, DE-A 37,32,654, U.S. Pat. No. 5,336,656, U.S. Pat. No. 5,256,387, U.S. Pat. No. 5,242,882 and U.S. Pat. No. 5,217,939 claim ceramic foams as carriers for catalysts, for example for stack gas treatment. With their random arrangement of webs, ceramic foams give substantially better mass transfer than extruded honeycomb elements which may have no webs in the flow direction owing to the extrusion technology, the pressure loss very advantageously being small. This applies in particular when the pore volume is more than 50%, more advantageously more than 70%, of the total volume of the catalyst carrier and the webs have thicknesses of less than 1 mm. Small pressure losses are particularly important in the application as carriers in stack gas treatment (DE-A 35,10,170), in vehicle exhaust gas catalysts (DE-A 37,31,888) or in the application as diesel exhaust gas filters (EP-A 312,501). Frequently, ceramic foams are also claimed as filters for the purification of very hot melts, such as metal melts (U.S. Pat. No. 4,697,632) or for the filtration of hot gases (EP-A 412,931).
All these applications make use of the preparation of open-cell foams by the infiltration of open-cell polymer foams. The inorganic materials claimed are just as varied as the applications. For foams having low thermal expansion, the material used is lithium aluminum silicate or cordierite. Such foams have particularly high resistance to large, abrupt temperature changes, as must be possessed by a catalytic converter for motor vehicle exhaust gases (JP-A 6,1295,283). On the other hand, inert behavior to metal melts is important for filtering metal melts. Here, .alpha.-alumina, silicon carbide and SiO.sub.2 and in particular mixtures thereof are used (EP-A 412,673). Foams of silicon carbide are particularly suitable for the filtration of iron melts or melts of iron-containing alloys (WO 88/07403). Silicon nitride, too, is claimed as filter material of ceramic open-cell foams (DE-A 38,35,807). EP-A 445,067 describes Y.sub.2 O.sub.3 stabilized zirconium oxide or ZrO.sub.2 /Al.sub.2 O.sub.3 mixed ceramics as filters for molten metals.
In addition to the infiltration of polymer foams with inorganic slips, followed by drying, pyrolysis and sintering, other methods for the preparation of inorganic foams have also been disclosed:
WO 95/11752 describes a process in which metals are chemically deposited on an open-cell polymer foam and, after drying and pyrolysis, an open-cell metal foam which can be converted into a ceramic foam by oxidation is obtained. Here too, drying and pyrolysis are very complicated. Drying and pyrolysis are avoided by the process which is claimed in EP-A 261,070 and starts from a metal foam, preferably from aluminum foam, for the preparation of ceramic foams and in which the latter is then oxidized to the metal oxide. A disadvantage of this process is that a metal foam has to be prepared beforehand in some manner. A process for the preparation of metal foams (Fraunhofer-Institut fur Angewandte Materialforschung, Bremen) starts from an aluminum powder, with which titanium hydride powder is mixed. The powder mixture is heated in a mold to just above the melting point of aluminum, the titanium hydride decomposing and the resulting hydrogen expanding the molten aluminum. In this case, which is not generally applicable, the melting point of the aluminum and the decomposition temperature range of the titanium hydride are compatible with one another.
In other known processes, too, hydrogen is used as a blowing agent for the preparation of inorganic foams: thus, it is known that strongly alkaline alkali metal silicates or alkali metal aluminates can be mixed with a powder of a non-noble metal, preferably aluminum, the metal dissolving and hydrogen being evolved as a gaseous blowing agent. After the foams have been dried, they have to be treated with ammonium compounds in order to remove disadvantageous alkali metal ions. After sintering, such foams contain less than 0.5% of alkali metal ions (EP-A 344,284, DE-A 38,16,893).
A dry process for the preparation of ceramic foams comprises mixing ceramic powders with volcanic eruption products which, when heated to 900-1400.degree. C., expand the resulting melt with gas evolution (JP-A 6,0221,371). Foams prepared in this manner are used in particular as heat-insulating (closed-cell) building material.
JP-A 2/290211 describes a process for the preparation of ceramic filters for metal melts, in which resin particles of various sizes, preferably of foamed polystyrene, are bonded to one another and the voids are infiltrated with a ceramic slip. After drying at 500-600.degree. C., the organic components are removed by pyrolysis and the foam is then sintered in air at 1200-1800.degree. C.
Open channels in ceramic foams can also be produced by applying short organic fibers, such as cotton, polyamide fibers or acrylic fibers, or inorganic fibers, such as graphite fibers, to an adhesive surface, applying further fibers with an organic binder, infiltrating the laid fiber web with inorganic slip, drying, pyrolyzing and sintering (EP-A 341,203). Foams having a pore volume of less than 35% are said to be produced in this manner. They are used as filters for molten metals.
Finally, it is also known that ceramic foams can be produced by adding aqueous polymer dispersions to aqueous ceramic slips, beating the mixture like cream, until it has from 1.5 to 10 times the initial volume, to give a foam, running the foam into a mold, drying, removing the organic assistants by pyrolysis and then sintering (EP-A 330,963). The amount by weight of organic material is 65-95% and the amount by weight of dispersion (dry matter) is 5-50%, which has to be removed by pyrolysis. What is disadvantageous for the applications of such open-cell inorganic foams is that relatively large air bubbles, too, are beaten in and that a large part of the foam cells are closed. Air is occluded during beating, and the resulting cells are stabilized by the polymer dispersion and only some of them break open during drying.
Problems arise in attempts to fill the reactive components of polyurethane foams to a high degree with inorganic powders and, by their reaction with one another, directly to produce an open-cell polyurethane foam which has a high filler content and from which the organic components can be removed by pyrolysis owing to the open-cell character. The molar mass of the components at the beginning of foaming is in fact so low that the foaming mixture is not sufficiently elastic, with the result that the small foam bubbles burst too early and the gaseous blowing agent, CO.sub.2, escapes substantially unused. The poor elasticity also rapidly results in cracks in the material, from which the gaseous blowing agent likewise flows away unused.